This invention relates to a ceramic dielectric material, and more particularly to a ceramic material having stable temperature coefficient of dielectric constant and suited for temperature-compensating capacitors.
Temperature-compensating ceramic capacitors find a wide use as circuit elements in communication equipments, color television receivers and so forth in order to compensate temperature characteristics of other circuit elements used in such equipments. For example, in LC circuits such as reasonance circuits or filter circuits, a temperature-compensating capacitor using a dielectric material of a constant negative temperature coefficient of a dielectric constant compensate a positive temperature coefficient of inductance so that frequency characteristics of such circuits remain constant in spite of change in atmospheric temperature.
However, it has been long desired that the aforesaid capacitors provide a high dielectric constant (.epsilon.), low dielectric loss (tan .delta.), and a temperature coefficient of a dielectric constant (TK.sub..epsilon.) maintained at a desired, constant value at varying temperatures. It follows from this that a dielectric material for use in a temperature-compensating ceramic capacitor is desired to have the aforesaid characteristics. Known as dielectric materials of this kind are compositions consisting essentially of any one of SrTiO.sub.3, CaTiO.sub.3, MgTiO.sub.3, CaZrO.sub.3, and the like. These materials however suffer from shortcomings, in that the value of a dielectric constant (.epsilon.) is as low as 30 to 16, in case the temperature coefficient of a dielectric constant (TK.sub..epsilon.) is set to a low value, and the temperature dependency of a temperature coefficient of a dielectric constant (TK.sub..epsilon.), i.e., the variation of TK.sub..epsilon. at varying temperature is as high as .+-.60 PPm/.degree.C, and in that, in case the temperature coefficient of a dielectric constant (TK.sub..epsilon.) is set to a high value, the temperature coefficient of dielectric constant (TK.sub..epsilon.) varies over a wide range at varying temperatures. In order words, the temperature-stability of TK.sub..epsilon. of such materials is poor.
There has been proposed attempts for overcoming these shortcomings. For example, ceramic compositions of MgO--La.sub.2 O.sub.3 --TiO.sub.2 system and MgO--Nd.sub.2 O.sub.4 --TiO.sub.2 system have been proposed in Journal of the American Ceramic Society, Discussions and Note, November, 1960, P609, and ceramic compositions of La.sub.2 O.sub.3 --TiO.sub.2 --MgO system have been proposed previously. These ceramic compositions achieve a success in providing almost zero temperature coefficient, with the accompanying some improvements in the temperature dependency of a temperature coefficient. These materials however still have little possibility of adjusting the temperatures coefficient to a desired value, depending on the intended use.
It is an object of the present invention to provide ceramic compositions having an improved temperature coefficient of a dielectric constant.
It is another object of the present invention to provide ceramic compositions capable of having an optional value of temperature coefficient of a dielectric constant with the temperature coefficient per se being stable against temperature variation.
It is still further object of the present invention to provide ceramic compositions of a high electric constant and low dielectric loss.